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Nuclear Medicine

E1044. Whole Lung Ventilation/Perfusion Mismatch

Albers B. Mayo Clinic, Jacksonville, FL

Address correspondence to B. Albers (albers.brittany@mayo.edu)

Background Information: Pulmonary ventilation/perfusion scintigraphy (V/Q scans) are frequently performed to evaluate for pulmonary embolus. However, both pulmonary ventilation and perfusion are affected by myriad disease processes which may result in abnormal V/Q scans. One particularly striking result is the unilateral, whole-lung absent or severely diminished perfusion. This is seen in approximately 2% of V/Q scans and can occur with normal or abnormal ventilation. If the ventilation is normal, there is a whole lung mismatched perfusion defect. According to the PIOPED-II Criteria this finding is technically classified as high-probability for pulmonary embolism if the chest radiograph is normal or the abnormality on chest radiograph involves less than the entire lung. However, only about 2% of pulmonary emboli present as a whole-lung mismatch perfusion defect, so alternative diagnoses should be considered. A matched defect, with abnormal perfusion and ventilation of the entire lung, is another uncommon finding with a slightly different differential diagnosis.

Educational Goals/Teaching Points: This exhibit reviews the key differential diagnosis for whole-lung mismatched and matched perfusion defects on V/Q scans. Illustrative cases include massive pulmonary embolus, fibrosing mediastinitis, severe fibrotic lung disease, iatrogenic pulmonary venous stenosis, and pneumonectomy. Correlative CT and MR images are included.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques: V/Q scans must be interpreted in the context of recent chest radiograph or CT. Many diseases which manifest as whole-lung matched ventilation/perfusion defects are obvious on the chest radiograph and include pneumonectomy, obstructive hilar mass, massive effusion, or pneumothorax. Mismatched whole-lung V/Q defects are more challenging as the diagnosis is often not seen on chest radiography, and further evaluation with CT or MR is typically necessary. Conditions that manifest as whole lung V/Q defects include massive pulmonary embolism, fibrosing mediastinitis, iatrogenic pulmonary vasculature stenosis, and unilateral pulmonary fibrosis.

Conclusion: Whole-lung perfusion anomalies should prompt evaluation for both pulmonary embolus and nonembolic phenomena. This has a fairly short differential diagnosis which can be refined by patient history and problem-specific imaging. Appropriate diagnosis directs the patient to appropriate care.